1. Field of the Invention
The present invention relates generally to regenerative thermal oxidizer systems for treating waste gas streams by oxidizing any volatile organic compounds that may be present. In particular, it relates to a system that uses wood waste as the fuel source for the oxidation reactions.
2. Description of the Related Art
Regenerative thermal oxidizer systems are useful for treating waste gas streams by destroying any volatile organic compounds that may be contained in such a waste gas stream. Such systems are increasingly important as ever stricter environmental regulations severely limit the levels of volatile organic compounds that may be released to the atmosphere.
It is well known to use regenerative thermal oxidizer systems with reversible gas flow in order to recapture heat that would otherwise be lost to the atmosphere. Regenerative thermal oxidizer systems generally include a heating regenerator containing refractory heat exchange material for preheating the gas stream to be treated, an oxidation chamber for oxidizing the volatile organic compounds or other combustible materials that may be present in the gas stream, and a cooling regenerator for cooling the oxidized gas stream before it is released to the atmosphere. Hydrocarbon fuels such as fuel gas or fuel oil are typically used as the fuel source that is supplied to the oxidation chamber to support the oxidation reactions.
After a period of time, the temperature of the heating regenerator starts to drop as heat is transferred to the gas stream. Similarly, the temperature of the cooling regenerator starts to rise as heat from the oxidized gas stream is transferred to the cooling regenerator. After a predetermined period of time has elapsed, and/or the regenerator temperatures reach predetermined limits, the flow of gas through the system is reversed so that the cooling regenerator is switched into heating service and the heating regenerator is switched into cooling service.
When the flow through a conventional two-regenerator regenerative thermal oxidizer system is reversed, an amount of untreated gas from the regenerator that was formerly in heating service passes through the system without being treated. This causes a spike in the emissions of volatile organic compounds that are released to atmosphere each time the regenerators are switched. In order to avoid this result, various modifications to regenerative thermal oxidizer systems have been developed. Some two-regenerator systems avoid spikes of emissions of volatile organic compounds during regenerator switching by using a system of bypass valves to purge the heating regenerator before it is switched into cooling service.
Another common modification uses three generators rather than two. In a three regenerator system the first regenerator is in heating service, the second regenerator is in cooling service and the third regenerator is in standby service. Once the temperatures of the heating and cooling regenerators have changed sufficiently, the cooling regenerator is switched to heating service, the standby regenerator is switched to cooling service and the heating regenerator is switched to standby service. While in standby service, the standby regenerator is purged of untreated gas by passing a flow of gas such as clean air or treated gas through that regenerator in order to drive any remaining untreated gas from it. The purged gas is generally recycled back to either the heating regenerator or directly to the oxidation chamber. An example of such a three-regenerator regenerative thermal oxidizer system is described in U.S. Pat. No. 5,026,277 to York.
Generally, it is desirable to use inexpensive and clean burning liquid or gaseous fuels such as fuel oil or natural gas as the fuel for the oxidation reactions in a regenerative thermal oxidizer. However, such fuels are not always readily available in all areas. Therefore, it is desirable in some circumstances to use an alternative fuel. Solid organic material such as wood waste in the form of hogged fuel, sawdust, planer shavings, sander dust or bark, can be inexpensive and readily available fuels that would be desirable for use as the fuel in a regenerative thermal oxidizer system.
Wood waste burners using such fuels are known and are currently used in the lumber processing industry. Such burners are often used as the energy source for lumber kilns. However, difficulties in handling solid fuels such as wood waste and the tendency for such fuels to produce solid combustion products in the form of ash or soot have generally made such fuels undesirable for use in regenerative thermal oxidizers.
An improved regenerative thermal oxidizer system is desired that is capable of cleanly and efficiently using solid organic fuel such as wood waste as the source of fuel.